TryStrToDateTime is this totally useless?

[BlackVoid]

FormatStr: 'yyyymmdd'; DateSeparator: #0; TimeSeparator: #0; IsDateTime: False

I call it with 20250525 and it fails (???).

I ended up using AI to write a totally new function that actually works.

[Thaddy]

If it "does not work" it is more likely you made an error, since TryStrToDateTime <> FormatStr...

[Fibonacci]

It appears that a separator is required

[Sieben]

TryISOStrTo* functions from unit DateUtils should work.

[tetrastes]

FormatStr: 'yyyymmdd'; DateSeparator: #0; TimeSeparator: #0; IsDateTime: False

I call it with 20250525 and it fails (???).

Of course it fails, as with your settings it will work with '2025'#0'05'#0'25'. #0 does not mean nothing. You have to learn pascal strings.
And from https://www.freepascal.org/docs-html/current/rtl/sysutils/trystrtodatetime.html:

If only one number is given, it is supposed to represent the day of the current month

So what do you think is your 20250525?

[munair]

So what do you think is your 20250525?

It is a valid (scientific) date representation, but it may need preformatting, which can be easily done with LeftStr/MidStr/RightStr if the representations are consistently 8 digits. Or use Copy() directly:

Code: Pascal  [Select][+][-]

1. DateStr := '20250525';
2.  
3. Year := StrToInt(Copy(DateStr, 1, 4));
4. Month := StrToInt(Copy(DateStr, 5, 2));
5. Day := StrToInt(Copy(DateStr, 7, 2));
6.  
7. ParsedDate := EncodeDate(Year, Month, Day);
8. Writeln('Parsed Date: ', DateToStr(ParsedDate));
9. Writeln('Formatted Date: ', FormatDateTime('yyyy-mm-dd', ParsedDate));

[tetrastes]

So what do you think is your 20250525?

It is a valid (scientific) date representation

Not for TryStrToDateTime with OP's Format settings. Note "your 20250525".

[Handoko]

I ended up using AI to write a totally new function that actually works.

You don't need to write a new function. TryStrToDate is working correctly. If you ask question in the here, you will learn more.

Below I write a demo following tetrastes suggestion:

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, StdCtrls;
9.  
10. type
11.  
12.   { TForm1 }
13.  
14.   TForm1 = class(TForm)
15.     Button1: TButton;
16.     Memo1: TMemo;
17.     procedure Button1Click(Sender: TObject);
18.   end;
19.  
20. var
21.   Form1: TForm1;
22.  
23. implementation
24.  
25. {$R *.lfm}
26.  
27. { TForm1 }
28.  
29. procedure TForm1.Button1Click(Sender: TObject);
30. var
31.   FormatStr:  TFormatSettings;
32.   DateTime:   TDateTime;
33. begin
34.  
35.   Memo1.Clear;
36.   FormatStr.DateSeparator   := #0;
37.   FormatStr.ShortDateFormat := 'yyyymmdd';
38.  
39.   Memo1.Append('Try with '''+'20250525'+'''');
40.   case TryStrToDate('20250525', DateTime, FormatStr) of
41.     True:  Memo1.Append('Correct');
42.     False: Memo1.Append('Incorrect');
43.   end;
44.  
45.   Memo1.Append(LineEnding);
46.   Memo1.Append('Try with ''2025''#0''05''#0''25''');
47.   case TryStrToDate('2025'#0'05'#0'25', DateTime, FormatStr) of
48.     True:  Memo1.Append('Correct');
49.     False: Memo1.Append('Incorrect');
50.   end;
51.  
52.  
53. end;
54.  
55. end.

[wp]

TryStrToDate uses the ShortDateFormat elements of the FormatSettings record to determine which parts of the date string correspond to day, month and year. And it uses the FormatSetting's DateSeparator to separate the date parts - there is no "no DateSeparator" setting, just counting the 'y', 'm' and 'd' characters in the format mask does not work...

A more flexible way to convert string to date is the function ScanDateTime in the DateUtils unit:

Code: Pascal  [Select][+][-]

1. var
2.   d: TDate;
3. ...
4.   d := ScanDateTime('yyyymmdd', '20250525');
5.